Double colon rules with phony prerequisites

[Fogle, Benjamin]

Hello,

I have a large project that includes libraries in Rust as dependencies. I want 
to run “cargo build” unconditionally, so it should use a phony target, but I 
only want to rebuild downstream binaries if a library actually changed. I found 
that double-colon rules seem to be an exception to “a phony target should not 
be a prerequisite of a real target file; if it is, its recipe will be run every 
time make goes to update that file” mentioned in the docs. (Though make only 
seems to reconsider timestamps the double-colon rule has a recipe.)

The following seems to work for me, but my question is: Am I missing something, 
and can I rely on this behavior?

.PHONY: build-rust
build-rust:
                cd rust-lib && cargo build

rust-lib/target/debug/libfoo.so:: build-rust
                @true

binary: binary.o rust-lib/target/debug/libfoo.so
                $(link-command)